The correct answer is C) waterside scale deposits. Treating boiler feed water to control hardness is necessary to prevent the formation of waterside scale deposits within the boiler. Hardness in water, typically caused by the presence of minerals like calcium and magnesium, can lead to the precipitation of these minerals on the boiler's internal surfaces, creating a scale buildup. This scale can insulate the boiler metal from the water, reducing heat transfer efficiency and potentially leading to overheating and damage. The other options are incorrect because: A) excessive feed water alkalinity is not the primary reason for treating boiler feed water, B) foaming is a separate issue that requires different treatment methods, and D) carryover, which is the entrainment of water droplets in the steam, is not directly related to the control of water hardness.